Expanding bootstrap accordian groups all at a time

Expanding bootstrap accordian groups all at a time

I am using bootstrap collapse element. When I expand one accordian group,
the other closes. What should do If i dont want the other accordian to
close when one if clicked?
<div class='accordion-group'>" + "<div class='accordion-heading'
style='font-size:15px;'>" + "<a class='accordion-toggle collapsed'
data-toggle='collapse' data-parent='#accordion1' href="#collapse1">
Thanks in advance :)